Sugarcane has been cultivated as a raw material for sugar, liquor, and the like for edible use. In addition, sugarcane has been used as, for example, a raw material for biofuel in a variety of industrial fields. Under such circumstances, there is a need to develop novel sugarcane varieties having desirable characteristics (e.g., sugar content, enhanced vegetative capacity, sprouting capacity, disease resistance, insect resistance, cold resistance, an increase in leaf blade length, an increase in leaf area, and an increase in stalk length).
In general, the following three ways may be used for identification of a plant variety/line: “characteristics comparison” for comparison of characteristics data, “comparison during cultivation” for comparison of plants cultivated under the same conditions, and “DNA assay” for DNA analysis. There are many problems in line identification with characteristics comparison or comparison during cultivation, including reduction of precision due to differences in cultivation conditions, lengthy duration of field research that requires a number of steps, and the like. In particular, since sugarcane plants are much larger than other graminaceous crops such as rice and maize, it has been difficult to conduct line identification based on field research.
In addition, in order to identify a variety resistant to a certain disease, an inoculation test is carried out using a causative microorganism of a disease after long-term cultivation of sugarcane, and then disease resistance data are collected by observing lesions and the like. However, transmission of the causative microorganism to an external environment must be securely prevented when the test is carried out, and thus it is necessary to provide, for example, facilities such as a large-scale special-purpose greenhouse, a special-purpose field or isolation facility from an external environment. Further, for creation of a novel sugarcane variety, first, tens of thousands of hybrids are created via crossing, followed by seedling selection and stepwise selection of desirable excellent lines. Eventually, 2 or 3 types of novel varieties having desired characteristics can be obtained. As described above, for creation of a novel sugarcane variety, it is necessary to cultivate and evaluate an enormous number of lines, and it is also necessary to prepare the above large-scale greenhouse or field and undertake highly time-consuming efforts.
Therefore, it has been required to develop a method for identifying a sugarcane line having disease resistance with the use of markers present in the sugarcane genome. In particular, upon creation of a novel sugarcane variety, if excellent markers could be used to examine a variety of characteristics, the above problems particular to sugarcane would be resolved, and the markers would be able to serve as very effective tools. However, since sugarcane plants have a large number of chromosomes (approximately 100 to 130) due to higher polyploidy, the development of marker technology has been slow. In the case of sugarcane, although the USDA reported genotyping with the use of SSR markers (Non-Patent Literature 1), the precision of genotyping is low because of the small numbers of markers and polymorphisms in each marker. In addition, the above genotyping is available only for American/Australian varieties, and therefore it cannot be used for identification of the major varieties cultivated in Japan, Taiwan, India, and other countries or lines that serve as useful genetic resources.
In addition, Non-Patent Literature 2 suggests the possibility that a sugarcane genetic map can be created by increasing the number of markers, comparing individual markers in terms of a characteristic relationship, and verifying the results. However, in Non-Patent Literature 2, an insufficient number of markers are disclosed and markers linked to desired characteristics have not been found.
Meanwhile, as a marker associated with disease resistance, a marker associated with black root rot resistance in sugar beet disclosed in Patent Literature 1 is known. In addition, a technique of selecting a Zea mays variety using a maker linked to a desired trait is disclosed in Patent Literature 2.
The level of infectiousness of the causative microorganism of sugarcane smut is high, and therefore the onset of smut quickly results in the infection of the entire field. Crops of sugarcane affected with smut cannot be used as raw material for sugar production, and even they die. Therefore, the development of smut will cause a significant decline in yield within the following year or later. Damage due to smut has been reported in more than 28 countries, including Brazil, the U.S., Australia, China, and Indonesia. Smut can be prevented by sterilization treatment prior to planting; however, preventive effects are limited to the period of early growth. Thus, cultivation of a sugarcane variety imparted with smut resistance has been awaited.